In the manufacture of pressure-sensitive recording papers, better known as carbonless copy papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor is normally coated on the back side of the front sheet of paper of a carbonless copy paper set. This coated backside is known as the CB coating. In order to develop an image or copy, the CB coating must be mated with a paper containing a coating of a suitable color developer, also known as dyestuff acceptor, on its front. This coated front color developer coating is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor.
Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dyestuff precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff, or dyestuff precursor, then reacts with the color developer in the areas at which pressure was applied, thereby effecting the colored marking. Such mechanism for the technique of producing pressure-sensitive recording papers is well known.
Among the well known basic, reactive, colorless chromogenic dye precursors used for developing colored marks when applied to a receiving sheet are such color developers as Crystal Violet Lactone, the p-toluenesulfonate salt of Michler's Hydrol or 4,4'-bis(diethylamino) benzhydrol, Benzoyl Leuco Methylene Blue, Indolyl Red, Malachite Green Lactone, spiro phthalide xanthenes such as 6"-(diethylamino)-3"-methyl-2"-(phenylamino)spiro[isobenzofuran-1(3H),9"-[ 9H]xanthen]-3-one, Rhodamine Lactone, and mixtures thereof.
Among well known color developers used on CF sheets are activated clays, zinc salicylate, and phenolic-type resins, such as acetylated phenolic resins, salicylic acid modified phenolics and particularly, novolac-type phenolic resins.
Various CF coatings and formulations have been used and various methods of applying the CF coating have been tried in the prior art. Aqueous CF coating compositions have been widely used, as for example in U.S. Pat. No. 3,672,935. However, when aqueous coatings are used, a large amount of water must then be evaporated, requiring large expensive dryers. Additional equipment must also be added to the coater to prevent sheet distortion, curl or cockle. The result is a coater and accompanying facilities which are expensive to build and operate.
It is also known that acidic color developer, such as phenolic resin, can be applied to the paper substrate as a solution in a volatile organic solvent which, after application, evaporates completely leaving a thin film of solid resin on the paper. This method is taught in U.S. Pat. Nos. 3,466,184 and 3,466,185. These coatings have several problems. Consistency of the solid solutions and the use of volatile solvents give rise to printing problems and cause swelling of rubber plates and rolls. They also have high energy requirements and present potential environmental contamination problems.
Because of these problems, the application of CF coatings is a costly and complicated operation if done separately. Most manufacturers coat the CF coating on the paper while the web is still on the paper machine to reduce the cost. Unfortunately, since most paper mills require large minimum orders, this approach is limited to high volume paper weights. A small user's volume is frequently too low to take advantage of this option.
A more economical approach, from the business forms manufacturer's perspective, is the application of the CF coating on the press with the rest of the printing and form preparation operations. Application of the CF coating as an ink would require no special equipment. Attempts have been made to develop such a system in order to take advantage of its benefits.
Maierson, U.S. Pat. No. 4,337,968, teaches dissolving a phenolic resin in ink along with modifiers to achieve the proper rheology. This type of CF ink solved the coating and economic disadvantages associated with volatile solvents and aqueous CF coatings. The CF ink could be applied by the standard business forms press on any paper stock with no modification and no sheet distortion.
While this represented a significant improvement over prior organic solvent based vehicles, the CF ink dried by adsorption and migrated into the sheet. The loss of color developer from the front surface of the sheet caused by the migration of the ink reduced the imaging ability of the coated sheet and also produced discoloration problems when placed in contact with CB coatings which typically contained about 5% free dye solution.
Special CB coatings that bind or trap the free dye solution were developed to overcome the discoloration problems. One example of this is Shackle et al, U.S. Pat. No. 4,063,754. In this coating, the capsules are produced in an aqueous vehicle and then flushed onto a wax vehicle, with evaporation of the water to produce a 100% solids, hot melt CB coating. The wax vehicle is selected so that any free dye solution will be bound up in the wax matrix.
However, this approach is believed to be undesirable both from an aesthetic and an economic standpoint. Waxcontaining coatings made in this way have a greasy, slick feeling to the touch. Moreover, to obtain an image density equivalent to that of an aqueous coating, a 100% solids coating must supply the same number of capsules as an aqueous coating.
Since approximately 60% of the solids, by weight, are inert vehicle components required for coatability, the hot melt coat weights are 2.5 (i.e., 1/0.4) times higher than the aqueous or solvent-based coatings. The additional solids merely add to the material cost of the CB coating. Further, there are additional processing costs associated with 100% solids coatings including energy costs for removing the water, equipment costs for the flushing and evaporation steps, and equipment and energy costs for the hot melt coater.
Accordingly, there remains a need for a non-volatile CF ink that can be press applied and which does not migrate into the paper stock after application obviating the need for special hot melt CB coatings.